Most modern vehicles include a vehicle radio that requires an antenna system to receive amplitude modulation (AM) and frequency modulation (FM) broadcasts from various radio stations. Many vehicle antenna systems include a dipole antenna that extends from a vehicle fender, vehicle roof, or some applicable location on the vehicle to receive these broadcasts. Improvements in vehicle antenna systems have included the development of backlite antenna systems, where antenna elements are formed on a rear window of the vehicle. The antenna elements in the backlite antenna systems are typically made of a conductive frit deposited on an inside surface of the window. Additionally, vehicle windshield antennas, such as the Solar-Ray antenna disclosed in U.S. Pat. No. 5,528,314, have also been developed. The Solar-Ray antenna includes a transparent conductive film laminated between the inner and outer glass sheets of the windshield. The windshield and backlite antenna systems provide a number of advantages over mast antenna systems, including no wind noise, reduced drag on the vehicle, elimination of corrosion of the antenna elements, no performance change with time, limited risk of vandalism, and reduced cost and installation.
Advancements in vehicle communication technologies has led to the need for various high frequency antennas to provide reception for different communication systems, such as radio frequency accessory (RFA) and key-less entry systems, cellular telephone, global positioning systems (GPS), personal communication systems (PCS), toll systems, garage door openers, etc. Because these antenna systems operate at higher frequencies than the AM and FM frequency bands, the size of the antenna is reduced from AM and FM antenna systems. These high frequency antennas must be positioned on a vehicle at a location where the antenna radiation is not adversely effected by the conductive vehicle body. It has been suggested to incorporate high frequency antennas in the vehicle windshield or backlite glass in combination with the existing AM/FM antennas. In one design, the high frequency antennas are mounted on an inside surface of the inside glass sheet of the windshield along a tinted top edge of the windshield so that they do not obstruct the view of the vehicle operator.
Each of the various vehicle window antennas typically include an antenna feed that is usually a coaxial cable connected to the antenna at a location suitable for optimum performance. The cable can be either directly connected to the antenna element or capacitively coupled to the antenna element through the vehicle glass. The center conductor of the cable is electrically connected to the antenna element and the outer conductor or outer shield of the cable is electrically connected to the vehicle ground, usually a vehicle body panel. An antenna design providing optimum performance would require that the outer shield of the coaxial feed cable be terminated and DC grounded to the vehicle body panel as close as possible to the edge of the window opening. This location provides a low impedance path for both DC and RF signals. However, antenna performance measurements indicate that the actual ground point for the feed cable can be located a distance of up to one-twentieth of the desirable reception wavelength S without significantly affecting the reception characteristics of the antenna.
In one known design, the DC ground for the Solar-Ray antenna is provided by soldering a metal bracket to the outer shield of the feed cable. The bracket is attached to the vehicle body by a ground screw about five inches from the window opening, or about S/25 of the middle of the FM frequency band. It has been determined that this type of feed arrangement would be unacceptable for proposed UHF and VHF antennas because of certain manufacturing concerns.
It is an object of the present invention to provide an alternate grounding system for these UHF and VHF vehicle antennas.